The present invention relates to a spacer for supporting a plurality of fuel rods forming a nuclear fuel assembly, and more particularly, it relates to an improvement in a spacer by using a specific zirconium alloy having a low neutron absorption cross-section and having good resiliency.
The spacer for supporting a large number of fuel rods forming the nuclear fuel assembly is means for retaining appropriate narrow gaps between adjacent fuel rods. Therefore, it is essential that the spacer prevent vibration in the radial direction of the fuel rods and bending in the lateral direction thereof during its retention of the fuel rods, and that no fretting corrosions are generated between the spacer and the fuel rods.
Various kinds of spacers have so far been proposed according to the different types of nuclear fuels and reactors. In a typical fuel assembly for boiling light water reactor (BWR) as shown in FIG. 1, for instance, a large number of fuel rods 1 are supported in such a manner that intermediate parts thereof are fixed by a plurality of spacers 2, while the upper and lower ends thereof are fixed by upper and lower fixing members 3 and 4 respectively. The spacers 2 are assembled by cross-linking a plurality of sheet divider members or grid sheets 7, so that they are formed into a grid constituting a plurality of spacer framework units within a peripheral band 6 surrounding a bundle of the fuel rods as shown in FIGS. 2A and 2B. The grid sheets 7 are embossed with bosses 8 as shown in FIG. 2B, while one corner of the spacer framework unit is fitted mechanically with a box-type four-sided leaf spring member 9 so as to support the fuel rod 1 elastically. Such spacer structure as described above is disclosed, for example, in U.S. Pat. No. 3,654,077. For the material of the spacers 2, zircaloy-4 is used for the peripheral band 6 and the grid sheets 7, while Inconel which has an excellent resiliency is used for the box-type four-sided leaf spring members 9, which are fitted mechanically to the grid sheets 7. This is because zircaloy-4 although having a low neutron absorption and an excellent corrosion resistance, has an inferior resiliency and, in addition, it cannot be welded to Inconel which has a superior resiliency. Accordingly, when the spacers are assembled, a complicated process is required to mechanically fit a number of box-type four-sided leaf spring members 9 to the grid sheets 7. Moreover, mechanical fitting makes it difficult to ensure the stability of the fitting and precision thereof, and further has the fault that a possibility of rupturing remains for the box-type four-sided leaf spring members while being employed in a reactor.